Gainesville FL Check Engine Light Problem

Before computers became common in automotive applications there were only two or three warning lights on your dash to let you know there was a problem under the hood- the alternator/generator light, the low oil pressure light and maybe an engine temperature light. These days, if you had a warning light for each thing that could go wrong under the hood, your dash would resemble the cockpit of a jet aircraft.

All modern cars are computer controlled. One of the first models to have computer controlled engine functions was the 1968 Volkswagen Type 3. It only had one computer that controlled only the fuel injection system. By the early 1990's almost all cars had one or two computers that controlled fuel injection, ignition and emission controls. These computers were as sophisticated those on the first lunar module. Over the last decade the technology has evolved rapidly to the point where some modern models like the Volkswagen Phaeton and BMW 7 series cars have anywhere from 64 to over 100 computers! Each computer monitors dozens of functions having to do with engine, emission control systems and many other vehicle systems.

When the engine management computer, known as the Power Train Control Module or PCM, detects one, or more of literally thousands of possible problems a single warning light is illuminate on your dash. This light is commonly known as the Check Engine Light or more specifically, the Malfunction Indicator Light (MIL). Depending on the vehicle manufacturer, it may also be called the Service Engine Soon Light.

On some vehicles the MIL is easily confused with the warning lights or text warnings on the Information Display that indicates regular maintenance, such as tune-ups or oil services, is due. Some manufacturers like Land Rover and Volvo have lights for regular engine maintenance that are often confused with the MIL. If you are unsure we will check it for you at no charge.

Another confusing factor is that there is that single MIL can be triggered by any of thousands of different problems. The fact that the light has been triggered means that the PCM has stored a five character alpha numeric code related to the type of problem that occurred. While we use this code to help diagnose the problem, it does not indicate the exact problem. Subsequent component testing is almost always necessary to prevent installing new parts needlessly.

The alpha numeric code is known as a fault code or Diagnostic Trouble Code (DTC). Under the present government regulations, or OBD II protocol, about 3,500 DTC can be defined.

When the MIL lights up we hook up a special computer (scanner or code reader) to your vehicle's PCM to retrieve the DTC from the PCM memory.

The fact that the MIL can indicate so many different problems can sometimes be confusing to the driver. For example, the vehicle could go to the shop 4 times for an MIL that was triggered by 4 entirely separate problems! So, it is important to make sure the shop records the DTC corrected on your invoice each time you have your vehicle scanned for an MIL. But, remember there can be a number of causes for any particular DTC.


A "monitor" is the aspect of the PCM's software that tests the system. It is important to remember is that DTCs are the result of a system or circuit failure but do not directly identify a failed component(s). Typically, what the DTC tells us is that the PCM is receiving "feedback" from a sensor or circuit that is outside a range of values the PCM is expecting to "see." Depending on the system monitor and DTC there can be any number of causes for an 'out of range' value. While the engine is running, throughout its various phases of operation, the PCM software instructs the computer to run a series of diagnostic tests, or "monitors" to test on various engine systems to detect various emission related faults in the vehicle. These "system monitors" detect faults by inference, not directly.

For instance, a system monitor may simply detect a voltage or signal from a particular component is "too low," "too high" or, otherwise "out of range." This does not necessarily mean the component itself is bad. There may be other problems contributing to the "out of range" signal the PCM is receiving. For example, a DTC for the oxygen sensor does not necessarily mean that the oxygen sensor is bad. There may be leak in the engine exhaust manifold or the intake manifold that is causing "to much air, or oxygen, to come in contact with the oxygen sensor causing an out of range value, thus triggering the MIL.

Types of System Monitors: Continuous, Non-Continuous System Monitors.

Part of the reason for the advent of the MIL is malfunctioning systems. After over a decade of government mandated emission controls, the EPA (Environmental Protection Agency) realized that despite the fact that emission control systems functioned properly when vehicles left the factory, unbeknownst to the drivers of those vehicles, there were many hundreds of thousands of vehicles on the road with faulty emission control systems contributing to atmospheric pollution. As a result, PCMs were given the task of not only controlling the vehicle emission systems but of monitoring the function of those systems, as well.

This is done through the use of "system monitors." There are seemingly countless system monitors that run tests on the various systems controlled by the PCM. Some of the system monitors operate continuously. These are called Continuous Monitors. Some of them run tests only once per trip. These are called Non-Continuous Monitors. In order for the system monitors to be run certain operating conditions, called 'enabling criteria,' must be met. This occurs during the drive cycle. Until these enabling criteria are met the system monitors cannot be run, and unless the system monitor is run a malfunction will not be detected, and until the malfunction can be detected the MIL will not be illuminated.

The system monitors that run continuously (whenever the engine is running) include: misfire detection, fuel trim, and the comprehensive component monitors. Continuous monitors can be of a more critical nature. For example, misfire detection, is critical because when an engine is misfiring the unburned gasoline, or hydrocarbon, reaching the catalytic converter causes it to overheat. A catalytic converter in this state can become "cherry red" hot and cause the vehicle to catch fire. An engine misfire will cause the MIL to flash and the engine should be shut off without delay. In addition to the fire hazard an engine misfire can in short order destroy the costly catalytic converter.

Examples of Non-Continuous System Monitors include catalyst and heated catalyst, oxygen sensor and oxygen sensor heater, evaporative emissions, secondary air, and Exhaust Gas Recirculation (EGR) system monitors.

Neither the Continuous, nor the Non-Continuous System Monitors can run their tests until the Enabling Criteria are met. Enabling Criteria are different and specific for each of the system monitors. Conditions that may be included in the Enabling Criteria are vehicle speed, engine rpm, engine load (the relation of throttle position to vehicle speed), throttle position, engine coolant temperature at start up and during operation, and time elapsed during operation. The term 'trip' describes the portion of a complete drive cycle that satisfies the Enabling Criteria of a particular monitor. A complete drive cycle contains the Enabling Criteria needed to run all the System Monitors. Sometimes it may take driving the vehicle more than once to achieve a complete drive cycle.

What if my Check Engine Light comes on again?

This is why the MIL will sometimes return a few days after having your car in for a Check Engine Light repair. When the PCM triggers the MIL it shuts down any further testing by the System Monitors. Once the MIL is cleared, the PCM resumes running the System Monitors. Remember, it may take a few days to get a complete drive cycle. That's why shortly after having an MIL repair done, the MIL can come on again. Often it is because there was more than one problem in the system, which was not detected because the MIL caused the PCM to stop running any further testing. If this happens after we do your repair, bring it in and we will 'read the code' in your PCM while you wait. When we read the code, make sure we show you the code and write it down for you. You can match this code with the one we documented on your repair invoice. If your Check Engine Light came back on within 2 weeks of your repair and it is the same code, we will do the next level of testing (scanning and component testing) for free. If we find a fault in the same part we replaced it is covered under our normal warranty. If the Check Engine Light was caused by another fault in the system we will advise you of any charge for that repair.

The next level of testing after reading the code

Once the system or circuit failure has been identified by retrieving the DTC, the technician must then manually perform further diagnostic tests using other specialized tools, such as digital volt ohm meters (DVOM), digital graphing multi-meters (DGMM) and digital storage oscilloscopes (DSO) to pin-point the precise problem that caused the of out of range feedback to the PCM. Out of range values can also be caused by faulty connectors and damaged or faulty wiring in the circuit. Retrieving the code is just the first step in diagnosing the problem. Reading the code is simple. Anyone can do it using a simple inexpensive 'code reader' but, to really find the problem it takes more specialized equipment in the hands of a knowledgeable, skilled technician. Unfortunately, the process is not as simple as just reading the code and replacing the indicated component.

There are various sensors that monitor the engine's performance. Oxygen sensors are used to monitor the engine exhaust gases. They send signals to the engine computer. Based on the amount of oxygen detected in the exhaust gases, the computer may vary the fuel mixture fed to the engine at a rate of up to 10 times per second at idle. Other sensors monitor pressure in the evaporative emissions system. This system recycles the fumes that come off the fuel in your tank. A check engine light could be triggered simply by leaving your gas cap loose! Unfortunately, you will have to pay the fee to have the vehicle scanned to retrieve the code pointing to a loose cap. The cap will have to be secured and the code cleared and Check Engine light reset. Anytime the computer detects a variation from the normal parameters it will trigger a check engine light. The scanners we use to retrieve the DTCs from your vehicle's computer and reset the check engine light range in price upwards of $8,000 to $12,000 and more, depending on scanner capabilities and associated "modules."

Unlike cars of the past, modern automotive engine systems do not require periodic adjustments. Your car's computer (also called Electronic Control Unit (ECU), Electronic Control Module (ECM) or Power train Control Module (PCM)) controls systems that only a short time ago were commonly adjusted as part of routine maintenance. You may recall hearing of adjusting your timing, fuel mixture and idle speed on older cars. Manual adjustments of these system are no longer possible. They are controlled exclusively by your cars computer. All this has been done to meet Federal Exhaust Emission and Fuel Economy regulations. Now, scheduled maintenance usually consists of just a spark plug set, fuel filter and air filter replacement. Tune-ups, as we knew them, are no longer necessary. Your car's computer monitors your engine performance, which is directly related to exhaust emissions and fuel economy. When the computer detects a problem, a light appears on your dash. Depending on the year of the vehicle light either says "Check Engine", "Power Loss", "Service Engine Soon", or the check engine symbol pictured at the top of this page.

When the engine is running and your check engine light is illuminated it is telling you that your vehicle's On Board Diagnostic System (OBD) has detected a malfunction in the engine management system and that a DTC is stored in the OBD computer. This DTC helps us determine what's wrong with your vehicle. It may point to one or more systems or circuits in your engine control management system for us to investigate.


Basic engine operation is like a three legged stool. If any of the three basic components is operating poorly or missing, the engine will not run properly or will not run at all. The three basic components that are required to make an engine run are: Spark- supplied by the Ignition System, Fuel- supplied by the Fuel System, and Compression- which exists as a result of the basic integrity of the metal parts of the area of the engine called the combustion chamber. Over time, the integrity of the metal parts of the engine called the combustion chamber deteriorate as part of normal wear.


The engine control system is comprised of a network of sensors and switches that send engine operating data to the Powertrain Control Module (PCM). The PCM makes adjustments to the operation of the engine based on the data received and instructions stored in the computer management software. Commands are then sent to three primary systems:

  • Ignition System
  • Fuel System
  • Emission Control System

Some of the sensors that may send data to the computer in your car are:

  • Mass Airflow Sensor (MAF) — senses the amount of air flowing into the engine.
  • Manifold Absolute Pressure Sensor (MAP) — senses the pressure in the intake manifold referenced to a perfect vacuum.
  • Barometric Pressure Sensor (BP) — senses the barometric pressure. Input from this sensor allows the PCM to recalibrate fuel delivery based on changes in altitude and weather.
  • Engine Coolant Temperature Sensor (ECT) — senses the temperature of the anti-freeze in the engine. This can allow the PCM to adjust fuel delivery and engine spark timing. For instance; when the engine temperature is cold, the PCM must increase fuel delivery to prevent the engine from stumbling when it is first started in the morning. When the engine is hot the PCM will retard ignition timing to prevent the engine from knocking, which may cause engine damage.
  • Intake Air Temperature Sensor (IAT) — senses the temperature of the air entering the engine. The PCM increases the amount of fuel the injectors spray for a lower temperature (denser) air, and decreases the amount of fuel sprayed into the engine for a higher temperature (less dense) air.
  • Throttle Position Sensor (TPS) — senses the position of the throttle plate.
  • Exhaust Oxygen Sensor (O2S) — senses amount of oxygen in the exhaust.
  • Crankshaft Position Sensor (CKP) — senses the position of rotation of the crankshaft.
  • Camshaft Position Sensor (CMP) — senses the position of rotation of the camshaft.
  • Vehicle Speed Sensor (VSS) — senses the rotation of the final drive. The PCM uses the VSS signal to control transmission shifts, the torque converter clutch and high speed fuel cut-off.
  • Power Steering Pressure Sensor (PSP) — senses when the power steering is in use, and combined with the engine speed sensor input, can allow the PCM to increase the engine speed to turn the power steering pump faster when maneuvering into parking spaces.
  • Brake Pedal Position Switch (BPP) — senses when the brake pedal is depressed to release the transmission torque converter clutch.
ODB II System
  • On Board Diagnostic Generation Two (OBD II) systems used in 1996 and later vehicles, the Check Engine Light is called a Malfunction Indicator Lamp (MIL). Federal legislation was passed to force vehicle manufacturers to use standardized codes, software parameters that determine when the MIL will be put on and a common computer serial plug to hook up to and scan your vehicle's computer. Prior to 1996 vehicle manufacturers saw developing proprietary connectors and codes as a way of forcing the customer back to the dealers for drivability and emissions repairs.
  • Here's how the system works. When the ignition switch is initially turned on and the engine is not running, the Malfunction Indicator Lamp (MIL) lights up so you may check that the bulb is not burned out. Once the engine is running, the MIL will light only if there is an emissions-related concern.
  • The OBD II system continuously monitors all engine and transmission sensors and actuators looking for electrical faults, as well as values that do not logically (rationally) fit with other power train data. When certain operating conditions are met and a comprehensive monitor detects a failure that will result in emissions exceeding a predetermined level, the computer stores a DTC, and illuminates the MIL.
  • The OBD II system also actively tests systems for proper operation while the vehicle is being driven. Fuel control and engine misfire are checked continuously. Catalyst efficiency, exhaust gas recirculation operation, evaporative system integrity, oxygen sensor response, and the oxygen sensor heaters are tested once per 'trip' when prerequisite operating conditions are met. The computer will illuminate the MIL if during these prerequisite operating conditions the system detects a failure that will result in emissions exceeding a predetermined level.
  • Whenever an engine misfire is severe enough, excessive amounts of unburned fuel will pass from the engine's combustion chamber into the exhaust system. When this happens the MIL will blink on and off. The unburned fuels, or hydrocarbons, that pass into the exhaust can cause damage to a platinum impregnated exhaust component called the catalytic converter. Catalytic converters are designed to "re-burn" the exhaust to reduce emissions, and are extremely expensive. When they become overheated due to the introduction of excessive hydrocarbons (unburned fuel) they will melt and block the exhaust system. This blockage will cause poor running and if bad enough will prevent the vehicle from running altogether. The symptom is similar to the high school prank of shoving a potato into a tailpipe to prevent a car from running.
  • When and how the Check Engine light is illuminated is determined by the your car's computer software parameters, which are determined by the Federal Government. OBD II regulations require the use of two different tests to detect the severity of a misfire and to determine the failure criteria to be used by the Power train Control Module (PCM). The two types of engine misfire are Type A and Type B. The Type A misfire (one-trip monitor) will set if the misfire condition is so severe that immediate catalyst damage will occur if the current engine speed, load and coolant temperature are maintained. If a type A misfire is occurring the PCM will begin flashing the MIL once per second, within 200 engine revolutions after the misfire is first detected. If the vehicle operating conditions change to ones that will not cause converter damage, the mil stops flashing and remains on steady. The Type B misfire (two-trip monitor) will set if a misfire of more than 2% is detected that could cause tailpipe emissions to exceed 1.5 times the Federal Test Procedure (FTP) standard. The PCM sets a "pending code" and stores engine data in freeze frame when a Type B DTC is set. If the same fault is detected on two consecutive trips under similar conditions, the MIL is turned on and a DTC is set. The MIL is also turned on if a misfire is detected during two nonconsecutive trips that are fewer than 80 trips apart.
  • Once lit, the MIL will remain on until the vehicle has completed three consecutive good trips (three trips in which the fault is not detected). The MIL is also turned OFF when stored diagnostic trouble codes are cleared. The MIL will only remain OFF once the fault is successfully repaired. On occasion, a problem in the computer may prevent clearing the code to the turn off the MIL. In which case, though the problem has been properly repaired, the light will remain on. This may indicate there is a problem with the circuitry or software inside the computer, and it may have to be replaced.
OBD I and Prior Check Engine Light

The emissions malfunction indicator on pre OBD II vehicles (most models prior to 1996) is known on some models as the "Check Engine", "Power Loss", "Service Engine Now", or "Service Engine Soon" light. Like OBD II systems, this light is intended to alert the operator when there is a failure in the system that may cause an increase of harmful emissions.

The light illuminates when the ignition key is in the ON position and the engine is OFF; again, like your oil and battery light this is to functionally test the system and check the bulb. When the light turns ON during engine operation, even momentarily, a system diagnosis is necessary to determine the fault.

When the light is ON steadily it means there is a fault currently detected. If the light illuminates and then turns off it can mean that the fault is intermittent or that the fault is only being detected intermittently. In either case, if the light is intermittent it usually means the technician will have to try to recreate the operating conditions under which the light illuminates in order to diagnose the problem. So, please take note of when the problem occurs so you can pass this information along to the service writer. Whether or not your shop will be able to scan the system on pre-OBD II systems will depend on how open that particular vehicle manufacturer designed the system. The ability to scan for codes varies greatly from vehicle make to make, on OBD I systems.

Diagnosis of an intermittent problem is frequently more difficult. This is sometimes a hit or miss situation. It often requires bringing the vehicle in several times until the problem can be duplicated and the fault is located, or just leaving the vehicle until that can be done. This warning light's purpose is to warn of increased emissions. However, in most cases if the system is not promptly repaired damage to other components can occur.

Our typical repairs and services can come with a 2-Year/24,000 mile local warranty and a 12 month/12,000 mile nationwide warranty.

We look forward to talking to you soon!

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